1. Field of the Invention
The present invention relates to a cathode active material which contains a complex oxide containing lithium (Li) and cobalt (Co), a method of manufacturing it, a cathode using the cathode active material, and a battery using the cathode active material.
2. Description of the Related Art
In recent years, portable devices such as a combination camera and a notebook personal computer have become widely used. Accordingly, small-sized secondary batteries with a high capacity have been increasingly demanded. The secondary batteries currently in use include nickel-cadmium batteries using an alkali electrolytic solution. However, the battery voltage thereof is 1.2V, which is low, and therefore it is difficult to improve the energy density. Therefore, it has been considered to develop so-called lithium metal secondary batteries using lithium metal. Such lithium metal has a specific gravity of 0.534, which is the lightest simple substance among solid simple substances. Further, the lithium metal has significantly poor electric potential and has the highest current capacity per unit weight among metal anode materials. However, in the lithium metal secondary batteries, there have been disadvantages as follows. That is, along with charge and discharge, lithium is grown on the anode dendritically, leading to lowered cycle characteristics. Otherwise, such grown lithium breaks the separator, causing internal short circuit. Therefore, secondary batteries in which a carbon material such as coke is used for an anode, alkali metal ions are inserted and extracted and thereby charge and discharge are repeated have been developed. In the result, deterioration of anodes due to charge and discharge has been reduced (for example, refer to Japanese Unexamined Patent Application Publication No. H10-333573).
In the lithium secondary batteries currently used generally, lithium cobaltate is used for the cathode, a carbon material is used for an anode, and the operating voltage is in the range from 4.2 V to 2.5 V. Regarding the cathode active material such as lithium cobaltate used for the cathode in such lithium secondary batteries operating at 4.2 V at maximum, only about 60% of the capacity is utilized to the theoretical capacity. Therefore, it is theoretically possible to utilize the remaining capacity by further increasing the charging voltage. In practice, it is known that a high energy density is realized by increasing the voltage in charging to 4.25 V or more (refer to International Publication No. WO03/197131). In particular, as a cathode active material, there are lithium nickelate, lithium manganate having a spinel structure and the like in addition to lithium cobaltate. Specially, lithium cobaltate is preferably used since lithium cobaltate can increase the electric potential most.
However, when the charging voltage is increased, there have been disadvantages as follows. That is, the oxidizing atmosphere in the vicinity of the cathode becomes strong. In the result, the electrolyte is easily deteriorated by oxidation decomposition, or cobalt is easily eluted from the cathode. Consequently, the charge and discharge efficiency is lowered, the cycle characteristics are lowered, and therefore it has been difficult to increase the charging voltage.
In the past, as a method for improving stability of cathode active materials, the following methods and the like have been reported. One method is that different elements such as aluminum (Al), magnesium (Mg), zirconium (Zr), and titanium (Ti) are dissolved (refer to Japanese Unexamined Patent Application Publication No. 2004-303459). Another method is that a small amount of a lithium-nickel-manganese complex oxide and the like is mixed (refer to Japanese Unexamined Patent Application Publication No. 2002-100357). Still another method is that the surface of lithium cobaltate is coated with lithium manganate having a spinel structure or nickel-cobalt complex oxide (refer to Japanese Unexamined Patent Application Publication Nos. H10-333573 and H10-372470).